The present invention relates to a method for the combinatorial preparation of material libraries and testing of the same by photoacoustic analytical methods and an apparatus for carrying out this process.
Photoacoustic methods are known for the analysis of substances in gases, liquids and solids. The physical effect of photoacoustics is based on the absorption of light energy by a molecule and the subsequent detection of the heat released by relaxation of the molecule, which can be measured as a pressure wave by microphonic measuring systems in a measuring instrument. The broad applicability of the method is based on the great number of molecular de-excitation processes which lead to a heating of the analyte and thus to a change in volume. The technical set-up for measuring the photoacoustic effect is, compared with other analytical methods, relatively simple and is associated with moderate equipment costs. The essential constituents of a measuring apparatus for determining photoacoustic signals comprise an excitation light source, a measuring device for recording the photoacoustic signals, preferably a microphone, and a data recording and analysis unit for analysing the signals produced, and if appropriate for controlling the light source and/or the measuring device.
With respect to the relevant prior art, reference is made to P. Remond, Applied Optics 1996, 35, No. 21, pp. 4065 to 4083 and to A. Bohren, Infrared Physics & Technology 1997, 38, pp. 423 to 435.
Using the photoacoustic effect, with a suitable measuring system and the currently known methods, even traces of substances can be detected down to the ppm range in gases and liquids within a few minutes of measuring time. Photoacoustics is thus one of the most sensitive and rapid optical analytical methods (cf. M. W. Sigrist, Optical Engineering, 1995, 34, No. 7, pp. 1916 to 1922).
For some time, combinatorial methods have been the centre of attention for research and development in the material research sector. By means of these combinatorial methods, as great a number as possible of different or identical chemical compounds are continuously being prepared and thus a material library established, which compounds are then studied for useful properties using suitable methods. In addition to magnetic, electronic, electromagnetic, optical, electrooptical and electromechanical properties, etc., catalytic properties of such materials prepared by combinatorial or highly parallel methods are also the centre of attention. In this respect, reference is made to WO 99/41005 and the prior art cited therein. This publication relates to arrays of heterogeneous catalysts and/or their precursors, made up of a body which has preferably parallel continuous channels and in which at least n channels contain n different heterogeneous catalysts and/or their precursors, where n has the value 2, preferably 10, particularly preferably 100, in particular 1000, especially 10,000. In addition, the publication relates to a process for preparing such arrays and a process for determining the activity, selectivity and/or long-term stability of catalysts in such an array.
U.S. Pat. No. 4,099,923 describes a substantially automated test unit for testing heterogeneous catalysts. The system described there consists of a tube-bundle reactor in which the same starting-material flow flows through each of the tubes. In the effluent of the tubes, the resultant products are passed via a multiway valve to a gas chromatograph and are there analysed for target products. The unit described there offers only a relatively small degree of parallelization, a system having 6 parallel tubes being given as an example. Obviously, in principle, a greater number of tubes is conceivable. However, the fact remains a problem that all gases in the described embodiment are passed for analysis via a multiway valve and thus sequential analysis of the samples takes place. If a higher degree of parallelization is wanted, this can be achieved by connecting further analytical instruments. However, this is associated with undesirably high capital costs.
WO 98/15969 and U.S. Pat. No. 5,959,297 describe the analysis of material libraries using translatable capillaries which take samples at defined places and pass these to an analytical instrument. In this case also, the same restrictions apply as for U.S. Pat. No. 4,099,923, this is a sequential processing of a finite number of samples in which the degree of parallelization can only be achieved by increasing the analytical capacity. For tests of particularly large numbers of building blocks from material libraries, this method is also not very suitable.
Relatively large material libraries can be simply investigated by analysing the heat formed in the reaction. In this case according to WO 97/32208 and WO 98/15813, using a thermosensitive camera, an entire material library can be studied for its useful properties. The disadvantage of the method is essentially that the thermosensitive camera reflects only the degree of activity of the catalysts via the heat being released. For a number of reactions this information is sufficient (total oxidations, complete hydrogenations, etc.) (see, inter alia, Holzwarth, A., Schmidt, H. W., Maier, W. F. Angewandte Chemie, 1998, 110, 19, 2788-2791; Reetz, M. T., Becker, M. H., Holzwarth, A., Angewandte Chemie, 1998, 110, 19, 2792-2795); in the case of other reactions, especially partial oxidations of hydrocarbons, to evaluate catalytic properties of a building block of a material library, information about the activity alone is not sufficient, since the selectivity of the building block in such types of reactions generally plays a greater role than the activity.
WO 99/19724 describes an optical method (REMPI) which makes it possible, by combining ionizing excitation of gases present in the effluent of a reactor with detection of the ionized molecules by one or more electrodes situated at the reactor outlet close to the ion production point, to detect selectively in parallel certain products in the effluent of a reactor or reactor array. Via this method, both the activity and the selectivity of the catalysts can be tested under conditions close to actual use. However, the method has a disadvantage due to the physical method of analysis. It is simple for a small number of organic molecules to produce characteristic ions which can be detected simply. However, with the majority of organic molecules, in each case very similar ion fragments are formed by the action of laser light under the conditions typical of this method. This prevents unambiguous detection of these molecules and unambiguous assignment of activity and selectivity parameters to the individual active compositions. A further disadvantage of the method is that when the analytical electrodes are operated for a relatively long period coking or polymer formation and thus destruction of the electrodes by the product gas must be expected.